In LTE (Long Term Evolution) Release-10, CA (Carrier Aggregation) was introduced to realize wide-band communication beyond 20 MHz (for example, communication at 100 Hz). In CA, communication is performed by using bundled CCs (Component Carriers) under a single radio base station eNB as illustrated in FIG. 7(a).
Afterwards, in LTE Release-12 and beyond, “Small Cell enhancement” was proposed, and “Inter-site CA” in which communication is performed by using bundled CCs (cells) under different radio base stations eNB has been studied for introduction as a more flexible network architecture than conventional ones (see FIG. 7(b)).
For example, as illustrated in FIG. 7(b), in a conceivable operation using “Inter-site CA” in which communication for a C-plane signal requiring reliability is performed in a cell #1 (a macro cell) under a radio base station eNB#1 (a macro eNB) via a SRB (Signaling Radio Bearer), while communication for a U-plane signal requiring wide-band communication is performed in a cell #10 (a small cell) under a radio base station eNB#10 (a small eNB) via a DRB (Data Radio Bearer).
In the case where the communication for the C-plane signal is performed in the cell #1 under the radio base station eNB#1 via the SRB, and the communication for the U-plane signal is performed in the cell #10 under the radio base station eNB#10 via the DRB as described above, the radio base station eNB#1 is expected to basically handle connections and settings between a mobile station UE and the radio base station eNB#1/the radio base station eNB#10, as illustrated in FIG. 8.
In this case, basically, it is expected that the cell #1 (the macro cell) is set as a reliable Pcell (Primary Cell), and the cell #10 is set as a supplementary Scell (Secondary Cell).
Here, according to the LTE specifications, a layer 2 is to be reset by notifying an RRC (Radio Resource Control) layer of an abnormality (a state mismatch), when detecting the abnormality.
Conceivable examples of the abnormality detected in the layer 2 are excess of a maximum number of retransmissions in RLC (Radio Link Control) and a failure in an RA (Random Access) procedure (excess of a maximum number of retransmissions of “RA preamble”).
Further, the radio base station eNB is configured to reset a layer 2 in a downlink by instructing the mobile station UE to perform “Intra-cell HO”, i.e., to perform a handover to the same Pcell (Primary cell).
Here, when the mobile station UE performs “Intra-cell HO”, the set Scells all change to a communication-disabled state (“deactive state”).
On the other hand, the mobile station UE is configured to reset the layer 2 in an uplink by performing a reconnection procedure.
In this case, the mobile station UE releases the present settings including the Scell and the like, and reestablishes a bearer (connection) for the radio base station eNB.